1. Field of the invention
This invention relates generally to the cultivation of plants under artificial light, and pertains more particularly to a commercially feasible method and apparatus for effectively controlling plant growth with the spectral characteristics of the lighting sources.
2. Description of the Prior Art
In growing plants, whether the culture be with soil or without soil, various factors must be favorable in order to encourage plant growth. The conditions such as supply of nurtrients, temperature, relative humidity and CO.sub.2 concentration in atmosphere must be favorable. Light conditions, of course, must be favorable, top.
Recently, there has been an increase in the number of greenhouses or plant factories. In these plant factories, plants are grown under natural lighting, artificial lighting or mixture of them. Among them, artificial lighting is most favorable to cultivate the plants efficiently and effectively in plant factories.
Metal halide lamps, high-pressure sodium lamps and fluorescent lamps are generally used in these plant factories.
Under such artificial lighting conditions, the growth of plants can be controlled by changing the spectral characteristics of the lighting sources used.
Not only the quantity but also the quality of the plants produced must be regarded as important. In the market, there is a large demand for qualities of vegetables. For instance, vegetables durable for transportation and soft vegetables for patients in hospital are usually demanded.
The number of photon incident on plants depends on the leaf area and the geometrical arrangements of leaves in the plant factory. Leaf areas and stem extensions are also important factors relating to efficiency in photosynthesis in plants. So, it is important to consider the photomorphogenesis in the plant factory.
It has been known that radiation between 400-700 nm in wavelength is effective for plant photosynthesis and that radiation longer than 700 nm in wavelength is effective for plant photomorphogenesis such as stem extensions, leaf areas and so on.
In past, however, such quantities as fresh weight or dry weight of plants have been considered in the mass production. The lighting sources for photosynthesis have been the main interest of development and the photomorphogenesis have been left out of consideration in the plant factory.